<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<script src="http://code.jquery.com/jquery-latest.js"></script>
</head>
<body>
    <select id='sel' name='x'>
        <option value='1'>a</option>
        <option value='2'>b</option>
        <option value='3'>c</option>
        <option value='4'>d</option>
    </select>
    <script type="text/javascript">
        $("#sel").val('3')
        //$("#sel").attr("value",'3');
    </script>
</body>
</html>